This invention relates to nuclear-reactor power apparatus and has particular relationship to the operation of nuclear reactors under water-solid conditions. In nuclear reactors of the pressurized water type, the coolant, usually water, is at a temperature and pressure near the critical temperature and pressure. Typically the temperature is about 580.degree. F. and the pressure in excess of 2000 pounds per square inch. The pressure is maintained by a pressurizer into which the coolant expands. The coolant flows in a loop through a steam generator which contains a fluid to which the coolant transfers heat. The fluid in the steam generator is here called secondary fluid.
Depending on the number of steam generators in nuclear power apparatus, there may be a corresponding or larger number of reactor coolant loops. There is usually only one pressurizer. During normal operation, the coolant in the pressurizer is at a predetermined water level and above this level there is a bubble or volume of vapor or steam which is essentially a pressure cushion. The desired pressure of the coolant is maintained by the vapor cushion. When the reactor apparatus is shut down, the pressurizer becomes filled with coolant, the coolant rises above a predetermined, highest measureable level confining the the bubble to a small volume. When the coolant is above this level, the nuclear-reactor power apparatus is said to be in a water-solid condition. This invention concerns itself particularly with the condition of the power supply apparatus during shut-down after the control rods have been inserted in the core and the steam has been or is in the process of being vented from the pressurizer. Under these conditions the apparatus components may be hot for a considerable time interval.
The permissible pressure of the coolant for a nuclear reactor is set by Appendix G to 10 CFR 50. Overpressure excursions which exceed this permissible pressure caused by malfunction or operator errors have been experienced during operation of the nuclear-reactor power apparatus in a water-solid condition. Such an overpressure excursion may arise from the addition of mass to the coolant. For example, the letdown valve may be isolated or closed while the coolant-charging pump or pumps continues to operate or the safety injection pump may be inadvertently put into operation. Such overpressure excursions may also occur by reason of addition of heat to the coolant. For example, the pressurizer heaters may be inadvertently energized. Or during shutdown, upon loss of residual heat removal capability, decay heat may be transferred from the core to the coolant. Or with the coolant pump started and a temperature difference between the warmer secondary fluid and the colder coolant, with the secondary fluid of the steam generator at a higher temperature than the coolant. Or between the warmer coolant and the colder coolant cold-leg loop seal, heat may be transferred from the warmer to the colder medium. The latter conditions which occur during cold shut-down of the nuclear-reactor power supply has been found to be a highly significant contributing factor to overpressure occurrences in a water-solid condition.
To mitigate the consequences of water-solid overpressurization of the coolant, the practice, in accordance with the teachings of the prior art, is to open the pressurizer power-operated or power-actuable relief valve. This valve is operated and when open relieves some of the coolant in the pressurizer. The practice in accordance with the teachings of the prior art is to apply the necessary power to open the valve. This practice has proven unsatisfactory. It has been found that the pressure increase in the coolant is not arrested sufficiently in all cases below the limit defined by Appendix G but that the pressure swings above this limit producing the undesired condition.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a method and apparatus in whose practice and use overpressurization in the water-solid state or condition of nuclear-reactor power apparatus shall be effectively suppressed.